An optical head device having an optical system provided with at least a light source (10), a shaping prism, an diffracting element (20) and a photodetector (60). The light source emits light whose intensity distribution has an elliptic shape in its cross section orthogonal to the optical axis. The shaping prism directs the light to a recording medium (70) after shaping the intensity distribution thereof into a circular shape. The diffracting element has diffracting regions formed by at least one division line substantially parallel to a reference direction. The light reflected from the recording medium forms an elliptic bright portion on the diffracting element. The photodetector is divided by a plurality of division lines (64, 65), at least one of which is parallel to the reference direction. When there is no focus error, the diffracting element diffracts the reflected light so as to converge it on the division line substantially parallel to the reference direction in the photodetector. For example, when the shaping prism is adapted to shape the elliptic intensity distribution with respect to its minor axis direction, the optical system is arranged so that the major axis of the bright portion is parallel to the division line of the diffracting element. Thus, it is possible to suppress offsets in focusing control due to wave-length variations of light.